If the Jedi were a manifestation of the Light Side of the Force, then the Sith would be their antithesis. While their origins may have fallen into the realm of legend and fairy tale, this much is known: the Sith were very real and were the bane of the Jedi for thousands of years. Millennia before the rise of the Republic, while the galaxy was still young and largely unexplored, with the sentient races of the galaxy struggling to find their place, a rift formed within the Jedi Order. This fracture, resulting in inevitable war divided the Jedi into two camps: those who chose to be servants in and of the Force, and those who would eventually become the Sith.
Leaving the known galaxy behind, these dissidents stumbled upon a region of space inhabited by a primitive peoples known as the Sith. The invaders were gods to these primitves, who they promptly subjugated to create a Sith Empire, adpoting the name for themselves. It was at this point that the Sith fell out of knowledge for thousands of years, expanding their empire to conquer more and more worlds in their secluded region of space. Generations of breeding with the primitive peoples resulted in a new pure-blood Sith race, strong in the Force and combining the use of the Dark Side and their own Sith magic.
While probably inevitable, it was by pure chance of luck that the Sith Empire regained contact with the galaxy proper. Seeing new worlds to conquer, the Sith embarked on a quest of domination known as the Great Hyperspace War in which the Sith would first clash with the Republic. It would not be the last. For the next six-thousand years new Sith emperors rose and fell as they vied for their own chance at galactic domination.
It was not until a farsighted Sith, in the aftermath of the death and destruction of a recent war changed the Sith ways forever. Again the Sith went into hiding, and for centuries the Jedi thought them extinct. However, the Sith were merely biding their time, striking from the shadows, and manipulating events across the galaxy that would ultimately bring down not only the Republic, but the Jedi as well. Seemingly, at last the Sith had finally conquered absolute. However, the circumstances that brought them to power would also prove to be their undoing as their fire was finally extinguished from the galaxy forever.